Alkaline dry batteries are currently widely used as the main power source for daily use articles, toys, hobby goods, game machines, portable music players, electronic devices, etc. Alkaline dry batteries are used in various environments, and may be used for as long as several years. Therefore, they are required to have high leakage resistance so that they should not leak even under a hot and humid environment for an extended period of time.
There are three possible routes through which electrolyte of common alkaline dry batteries leaks. These routes of electrolyte leakage are described with reference to FIG. 6, which is an enlarged cross-sectional view of the main part of the seal portion of a typical AA-size alkaline dry battery.
In the battery of FIG. 6, the open edge of a positive electrode case 31 is bent inward and in an arch to form a crimp 31a, so that a seal unit 39 is attached to the opening of the positive electrode case 31 to seal the opening. The seal unit 39 is prepared by welding a negative electrode current collector nail 36 to the center of a negative electrode terminal plate 37 in advance, and forcefully inserting a body 36a of the negative electrode current collector nail 36 through the opening (not shown) of a central cylindrical part 35a of a resin seal member 35 for integration. At this time, an outer peripheral cylindrical part 35d of the resin seal member 35 is sandwiched between the negative electrode terminal plate 37 and the positive electrode case 31.
In this alkaline dry battery, the first route of electrolyte leakage is a route of leakage of alkaline electrolyte between the inner face of the open edge of the positive electrode case 31 and the outer surface of the outer peripheral cylindrical part 35d of the resin seal member 35. However, the leakage through this route has been addressed by the development of various sealants and the research into rigid seal structure, so that commercially available dry batteries now have high reliability.
The second route is a route of leakage of alkaline electrolyte between the body 36a of the negative electrode current collector nail 36 and the central cylindrical part 35a of the resin seal member 35. The negative electrode current collector nail 36 is forcefully inserted through the opening of the central cylindrical part 35a, so that the alkaline electrolyte is sealed in.
When the negative electrode current collector nail 36 is loosely fitted to the central cylindrical part 35a, the alkaline electrolyte creeps up therebetween (hereinafter also referred to as “creeping”), thereby resulting in leakage. On the other hand, when the fit therebetween is too tight, the central cylindrical part 35a becomes cracked due to stress, and thus the seal is broken, thereby resulting in leakage of the alkaline electrolyte. When the resin seal member 35 is made of polyamide, in particular, it tends to become cracked since it is hydrolyzed by the alkaline electrolyte and deteriorates with time.
To prevent leakage through the second route, various proposals have been made (e.g., Japanese Laid-Open Patent Publications No. 2007-80574 and No. Hei 8-124544), but there is a need for further improvements.
As for the third route, a part of the connecting part 35c of the resin seal member 35 becomes cracked and broken due to stress, and the alkaline electrolyte leaks through the broken portion. This phenomenon tends to occur particularly in the thinned section 35e, since the stress or strain applied to the outer peripheral cylindrical part 35d of the resin seal member 35 to attach the seal unit 39 to the opening of the positive electrode case 31 tends to concentrate in the thinned section 35e. When the resin seal member 35 is made of polyamide, in particular, it tends to break since it is hydrolyzed by the alkaline electrolyte and deteriorates with time.
To solve this problem, for example, Japanese Laid-Open Patent Publications No. 2005-79021 and No. Hei 10-162800 disclose techniques of modifying the shape or structure of the seal unit 39 in order to reduce the transmission of stress or absorb stress. According to these conventional techniques, for example, the connecting part is provided with a redundant structure and thus some play, in order to prevent the stress created to attach the seal unit from being exerted on the connecting part.
However, alkaline dry batteries, which are widely used as the main power source for the above-noted various devices, are required to have not only leakage resistance but also higher capacity in order to provide longer battery life. It is thus necessary to pack a larger amount of power generating elements into the battery case and reduce the size of the seal unit. Therefore, providing the connecting part with a redundant structure, as in the aforementioned conventional techniques, not only increases the material costs but also becomes an obstacle to heightening the capacity of the dry battery.
As discussed above, many technical problems remain unsolved with respect to the leakage resistance of alkaline dry batteries, in particular, the leakage through the second and third routes.
The invention has been made in view of the above-discussed problems and aims to provide a leak-proof alkaline dry battery, in which alkaline electrolyte hardly leaks between the central cylindrical part of the resin seal member and the negative electrode current collector even during long-term storage, and leakage resulting from stress cracking and breakage of the thinned section of the resin seal member is prevented.